oe.classutils: add module

This adds a ClassRegistry utility metaclass, as maintaining a class registry
is a fairly common thing to do.

(From OE-Core rev: 6a46c375bea03b145bea41ec29ae2fd4d3cd9db6)

Signed-off-by: Chris Larson <chris_larson@mentor.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/lib/oe/classutils.py

@@ -0,0 +1,43 @@
+class ClassRegistry(type):
+    """Maintain a registry of classes, indexed by name.
+
+Note that this implementation requires that the names be unique, as it uses
+a dictionary to hold the classes by name.
+
+The name in the registry can be overridden via the 'name' attribute of the
+class, and the 'priority' attribute controls priority. The prioritized()
+method returns the registered classes in priority order.
+
+Subclasses of ClassRegistry may define an 'implemented' property to exert
+control over whether the class will be added to the registry (e.g. to keep
+abstract base classes out of the registry)."""
+    priority = 0
+    class __metaclass__(type):
+        """Give each ClassRegistry their own registry"""
+        def __init__(cls, name, bases, attrs):
+            cls.registry = {}
+            type.__init__(cls, name, bases, attrs)
+
+    def __init__(cls, name, bases, attrs):
+        super(ClassRegistry, cls).__init__(name, bases, attrs)
+        try:
+            if not cls.implemented:
+                return
+        except AttributeError:
+            pass
+
+        try:
+            cls.name
+        except AttributeError:
+            cls.name = name
+        cls.registry[cls.name] = cls
+
+    @classmethod
+    def prioritized(tcls):
+        return sorted(tcls.registry.values(),
+                      key=lambda v: v.priority, reverse=True)
+
+    def unregister(cls):
+        for key in cls.registry.keys():
+            if cls.registry[key] is cls:
+                del cls.registry[key]